Gigabit Ethernet is an increasingly popular communication standard, with a nominal data rate of 10 Gbit/s. One form of 10 Gigabit Ethernet is IEEE 10GBASE-T, used to provide 10 gigabit per second connections over unshielded or shielded twisted pair copper wires. The wire-level modulation for 10GBASE-T is a Tomlinson-Harashima Precoded (THP) version of pulse-amplitude modulation with 16 discrete levels (PAM-16), encoded in a two-dimensional constellation pattern known as 128-DSQ (Double Square QAM).
Tomlinson-Harashima Precoding (THP) is a scheme in which the channel equalizer resides in the link transmitter, theoretically enabling the link receiver to see “perfect” symbols, as the transmit equalizer precompensates for impairments in the channel. The equalizer tap coefficients are generally calculated by the link partner receiver in accordance with, for example, a “Least Mean Squares” (LMS) algorithm during an initialization training period. The calculations made by the receiver theoretically provide an optimum set of coefficients that best characterize the inverse transform of the detected channel characteristics. Once the filter tap coefficients are calculated by the receiver, the values are communicated back to the transmitter and set until the next training sequence (which may never happen unless the link is intentionally broken). This set and forget methodology is mandated by the current 10GBASE-T Specification by IEEE 802.3an.
While the proposed 10GBASE-T transmission scheme works well for its intended applications, the fixed “set and forget” optimum THP coefficient generation scheme may be problematic in some circumstances. For example, in environmental conditions where heating variations occur, cable lengths may change by small amounts. Such small changes may potentially cause offsets to signal sampling phases that were calculated and optimized during initialization. Slight variations in sampling phase may have dramatic effects in signal-to-noise-ratio (SNR) due to aliasing effects in and around the Nyquist frequency (half the baudrate).
Accordingly, systems and methods are needed that provide the capability to generate THP coefficients that have improved robustness over a wider range of operating conditions.